Thermally activated sulfurization of In-Cu bilayers for CuInS2 solar cells

Sulfur vapor or H2S is often used for the fabrication of CuInS2 thin film solar cells by sulfurizing Cu/In bimetallic layers at elevated temperature over 400 °C. In order to promote low temperature sulfurization and/or faster sulfurization, sulfur vapor was thermally activated before it reaches Cu/In metallic precursor layers for the formation of CuInS2 thin films. The thermal activation of sulfur at over 600 °C dissociates high molecular weight sulfur molecules primarily comprised of S8 into smaller size molecules. Thermodynamic calculation predicted more spontaneous sulfurization process with S2 molecules. However, experimental results indicated that more complex phase transformation process was occurring with the thermally activated sulfur molecules. While In-rich Cu/In bimetallic precursor layers only experienced minor compositional change in Cu/In ratio after sulfurization, Cu-rich films have lost significant amount of In resulting increased Cu/In ratio after sulfurization process. The compositional change of the Cu-rich bilayers was sensitive to the temperature at which sulfur molecules were activated (dissociated). In general, more indium loss was observed with a higher activation temperature. The Cu/In ratio was reduced down slightly below 1.0 by KCN etching of the sulfurized CuInS2 films indicating the existence of CuS phase and also indium binary phase, both observed in XRD patterns.